Several methods and apparatuses for inspecting containers and the contents thereof are known in the art. Particularly in the field of manufacture and distribution of medicaments the tolerances regarding the number of contaminated entities or imperfect containers are zero or at least very low. Hence the requirements regarding, e.g., inspection systems for such manufacture processes are high, and there is often a noticeable dependency between the quality of the inspection system, especially its false rejection rate, i.e. the number of flawless entities falsely rejected, and the profit obtainable from the process.
Regarding inspection of substantially transparent liquids contained in transparent containers, several disclosures of particle detection by measuring the amount of light penetrating the container exist. U.S. Pat. No. 3,598,907 and U.S. Pat. No. 3,777,169 thus disclose illuminating a transparent liquid-filled container from beneath, and filming it from the side while the liquid, and any foreign particles, rotate around its vertical axis. The illumination is, however, not very good in the top end of the container opposite to the illumination source, and especially for contents not fully transparent, e.g. suspensions or more or less opaque liquids, the detection rate is low. U.S. Pat. No. 4,095,904, U.S. Pat. No. 4,136,930 and U.S. Pat. No. 4,472,745, and European patent application no. EP 0 293 510 A2, disclose illuminating a transparent liquid-filled container from a direction opposite to the filming or detection direction, i.e. the container is illuminated from behind, and the measured amount of light penetrating the container is used to determined the existence of any foreign particles in the liquid. This method is, however, not optimal, and especially not for contents not fully transparent, e.g. suspensions or more or less opaque liquids, as the liquid or suspension serve as a light diffuser to the illumination, causing the picture of any particle to be so blurred that it may not be detected.
PCT application WO 92/14142 discloses an inspection apparatus where the liquid-filled container is illuminated from an angle between 90 degrees and 180 degrees relative to the direction of detection, and where the detection is made by means of a vertical line scanner. The liquid and any foreign particles therein, rotate around the container's vertical axis, while the container itself is fixed. This inspection apparatus is capable of detecting particles in transparent and, to some degree, more or less opaque liquids, e.g. suspensions. The illumination method of that application has however proved to be non-optimal due to limited optical dynamic range, as, e.g., a suspension acts as an ideal diffuser to the illumination, thereby causing any particle to be very uniformly illuminated and thereby avoids shadows. A particle is thus only detectable if the difference between its colour, e.g. light grey for glass particles and dark grey for opaque particles, and the colour of the diffuse, medium grey background is sufficiently significant.